Geological testing system relies on ballscrew

To control the pressure exerted on the rock or soil samples, UK manufacturer of computer controlled testing systems for geological engineers, GDS Instruments, relies on a precision ballscrew from SKF

Soil testing is a precise science.

Usually performed in a laboratory, it relies on some relatively simple mechanics in the heart of the testing equipment, to create the testing conditions required.

One of the UK's leading designers and manufacturers of computer controlled testing systems for geological engineers is GDS Instruments Limited.

To control the pressure exerted on the rock or soil samples, the company relies on a precision ballscrew from SKF.

The essence of the GDS automated triaxial testing system is a pressure volume vessel in which a sample is suspended in a fluid medium, usually water, but very occasionally oil or air.

Radial loading is applied to the sample by increasing the fluid pressure in the cylinder using hydraulic actuators.

These are driven via the SKF ballscrew using a stepping motor.

Load is also applied mechanically from below and further hydraulic pressure is applied to within the sample.

By using accurate hydraulic cylinders and the SKF ballscrew, the pressure can be calibrated according to each step of the controlling motor.

An encoder feedback from the motor gives the position of the rotor and thereby closes the control loop.

The accuracy of the volume change is as fine as 1mm3 per step of the motor.

The accuracy of both the cylinder and also the pitch of the SKF ballscrew therefore determine the accuracy of the volume and hence the pressure applied.

There is also a pressure transducer fitted to the hydraulic cylinder to give a feedback record of the actual pressure within the actuator.

The testing system is easy to set by a keypad on the top of the actuator Housing.

A computer within the unit both controls and records the measured data, which can be viewed in near real-time on the PC or downloaded via a standard RS232 interface.

On line graphics display up to three graphs, together with a block of current live test data.

The GDS triaxial system is capable of measuring radial stress, back pressure, test specimen volume change, cell volume change, pore water pressure, axial force and axial displacement.

From these measured parameters a host of other parameter values can also be derived.

Three pressure actuators are incorporated in each triaxial system, but GDS also supplies individual actuators for those configuring their own systems or for laboratories that simply want a reliable means of creating an accurately controlled variable pressure.

GDS engineers selected the SKF ballscrews because they offered the best combination of accuracy, reliability, long life and cost.

The components have never let GDS down in spite of being subjected to high loads.

Approaching 95% of GDS production is exported, with most systems going to research institutions and bodies such as the British Geological Survey.

Other users include mining and quarrying research centres, oil and gas exploration and pharmaceutical laboratories.

GDS makes testers up to a frame type pressure vessel capable of handling pressure up to 250kN!.

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